Accurate determination of skin type (or skin status) is important in terms of providing effective skin care for maintaining healthy skin. Consequently, when providing skin care using cosmetics, for example, the skin type of a cosmetic user is evaluated through the asking of questions and so forth by a beauty technician. In addition, skin status or function is evaluated according to observed or measured parameters using various types of measuring instruments for the purpose of objectively evaluating skin type.
Research has recently been actively conducted on stratum corneum oxidized proteins in the relationship between aging accompanying aging of the skin and photoaging. Oxidized proteins consist mainly of proteins into which carbonyl groups have been introduced as a result of being subjected to oxidation, and in general, consist of those formed as a result of the NH2 groups of amino acid residues such as Lys, Arg and Pro in proteins being oxidized directly resulting in their conversion to carbonyl groups, and those formed as a result of lipid being oxidized to lipid peroxides, which in turn are degraded into highly reactive aldehydes that bind with protein. As a result of extensive research relating to aging, oxidized proteins are recognized to increase with aging (brain, liver, fibroblasts), in Alzheimer's disease, in progeria (Werner's syndrome), and the like.
In the skin, sebaceous lipid on the skin surface is oxidized by free radicals resulting in the formation of lipid peroxides. The formation of lipid peroxides initializes the oxidation of proteins, and once these lipid peroxides are formed, oxidation proceeds continuously, not only causing irritation on the skin surface, but also penetrating deep into the stratum corneum and damaging cells. Thus, by evaluating the properties of oxidized proteins of the skin, such as the amount present and their distribution state, and determining the skin type or skin status, those evaluation results can be expected to be used in some embodiments for determining the direction of subsequent skin care and selecting cosmetics. At present, however, although there is predicted to be some kind of vague relationship between oxidized proteins and skin damage, the manner in which the oxidation of stratum corneum protein actually affects the properties of the skin has yet to be determined.